1. Field of the Invention
The present invention is related to methods and apparatus for improving distributions of both spent catalyst and transport gas into a regenerator of a fluid catalytic cracking unit.
2. Description of Related Art
In a typical Fluid Catalytic Cracking Unit (FCCU) consisting of a regenerator, a riser reactor and a stripper, such as that shown in U.S. Pat. No. 5,562,818 to Hedrick which is incorporated herein by reference, finely divided regenerated catalyst is drawn from the regenerator through the regenerator standpipe and contacts with a hydrocarbon feedstock in a lower portion of a reactor riser. Hydrocarbon feedstock and steam enter the riser through feed nozzles. The mixture of feed, steam and regenerated catalyst, which has a temperature of from about 200xc2x0 C. to about 700xc2x0 C., passes up through the riser reactor, converting the feed into lighter products while a coke layer deposits on the surface of the catalyst, temporarily deactivating the catalyst. The hydrocarbon vapors and catalyst from the top of the riser are then passed through cyclones to separate spent catalyst from the hydrocarbon vapor product stream. The spent catalyst enters the stripper where steam is introduced to remove hydrocarbon products from the catalyst. The spent catalyst then passes through a spent catalyst transfer line to enter the regenerator where, in the presence of air and at a temperature of from about 620xc2x0 C. to about 760xc2x0 C., the coke layer on the spent catalyst is combusted to restore the catalyst activity. The regenerated catalyst is then drawn from the regenerator fluidized bed through the regenerator standpipe and, in repetition of the previously mentioned cycle, contacts the feedstock in the lower riser.
Catalyst regeneration is a critical step in FCCU operation. The success of the step depends on the contacting efficiency between the spent catalyst and oxygen-containing gas in the regenerator. While the operation of an FCCU with a single catalyst inlet opening was acceptable for many years, the potential benefit of improving catalyst distribution in the regenerator has become apparent more recently. An ideal condition for catalyst distribution is that the time for distribution and mixing of catalyst should be less than that for coke combustion. As the regenerator diameter increases, the radial mixing time of catalyst becomes longer. At the same time, as the regeneration temperature increases, the time required for combustion becomes shorter. Hence, the benefit of improving spent catalyst distribution is more significant for an FCCU comprising a regenerator vessel of large diameter or in which regeneration is conducted at higher temperature.
Another important aspect of the spent catalyst distribution is to control afterburn, which is characterized by substantial temperature increase in the dilute phase of the regenerator. If the transport gas, most commonly air, coming along with the spent catalyst is not well distributed, gas will form large bubbles at the discharge of the spent catalyst distributor, rising quickly through the dense fluidized bed with little time for combustion, and releasing oxygen-rich gas into the dilute phase. This leads to afterburn and poor combustion efficiency of the transport gas in the dense bed.
There are a number of prior art devices using various designs to improve spent catalyst distribution in the regenerator. FIG. 4 shows a schematic drawing of a prior art spent catalyst distributor used by Assignee. It shows that spent catalyst is discharged through a plurality of discrete slots 43 on the side walls at the upper end of a spent catalyst riser 10. One shortcoming of this distributor is that catalyst exits the slots with little radial velocity, leading to insufficient catalyst distribution in the regenerator. Another shortcoming is that transfer gas is just being separated from the catalyst at the riser top and has little time to re-mix with catalyst, leading to poor usage of the transport gas for regeneration and more afterburn.
U.S. Pat. No. 4,595,567 discloses devices for distributing catalyst into an FCC regenerator in the form of an air/catalyst distribution grid at the upper end of the spent catalyst riser, which comprises a plurality of discrete openings, such as nozzles, along the length of sections of a radially extending grid. It is known that this type of distributor grid is prone to erosion damage, as discussed at p.145 of xe2x80x9cFluid catalytic Crackingxe2x80x94Technology and Operationxe2x80x9d by J. W. Wilson.
U.S. Pat. No. 4,150,090 discloses a device comprising a spent catalyst riser axially located at the center of the regenerator, terminated by a plurality of radially extending fluidized catalyst distributor troughs discharging catalyst near the surface of the regenerator bed. The trough has a substantial U-shaped cross-section and the bottom of the trough slopes downwardly with fluidization gas injection along the length of the trough. U.S. Pat. No. 5,635,140 discloses an improvement over U.S. Pat. No. 4,150,090 with similar distribution troughs but with the improvement being that the troughs are self-aerated.
U.S. Pat. No. 5,156,817 discloses a device for supplying catalyst at the upper end of a spent catalyst riser through a plurality of channels confined by inverted v-shaped members. Catalyst is discharged downwardly along the length of the channels which are closed at their proximal end. The apparatus comprises a plurality of channels of different lengths and emanates in a fan formation from a single supply conduit, with the longest channel covering almost the diameter of the regenerator.
U.S. Pat. No. 5,773,378 discloses a device to distribute spent catalyst at the lower end of a spent catalyst standpipe. The standpipe enters the regenerator from the side wall, near the top of the bed level, conveying the catalyst through a horizontal conduit to the center of the regenerator, followed by a vertically downward conduit with a deflector plate end cap, and discharging catalyst through a plurality of discrete radial slots on the lower side wall of the vertical conduit.
EP patent 0622,116, B1 discloses a device which distributes spent catalyst by means of a central spent catalyst riser terminated with a junction connecting to multiple, horizontal conveying conduits and discharging catalyst at the ends of the horizontal conduits to discrete distribution points.
The major shortcomings of these prior art spent catalyst distributors include:
Incomplete coverage by discrete dischargexe2x80x94The prior art spent catalyst distributors use either a plurality of distribution arms or discrete slots from a single source of spent catalyst. In either case, the initial distribution of spent catalyst in the regenerator leaves some areas uncovered between distribution arms or discrete slots.
Poor distribution of transport gasxe2x80x94Prior art spent catalyst distributors pay little attention to the distribution of transport gas. This leads to poor utilization of transport gas for regeneration and more afterburn.
Bulky mechanical structurexe2x80x94Prior art spent catalyst distributors have long horizontal arms which are not reliable in the turbulent environment of the regenerator. Their bulky structure also makes them difficult to fit into an existing regenerator for retrofit.
It is an objective of the instant invention to improve spent catalyst distribution in the regenerator. Another objective is to simultaneously improve the distribution of the transfer gas in the regenerator. Yet, another objective is to achieve such distributions through a simple mechanical apparatus that is compact, robust and easy to implement in an existing FCCU.
The present invention is related to methods and apparatus for improving distributions of both spent catalyst and transport gas into a regenerator of a fluid catalytic cracking unit. Spent catalyst and transport gas move upwardly through a spent catalyst riser and are diverted in a radially outward direction by a deflector cone. The catalyst and transport gas are re-mixed as they move radially outward between two disks before discharging from the outer edges (perimeter) of the distributor into the regenerator in a substantially uniform radial direction. The distributor is adapted to provide continuous discharge from its perimeter so as to cover the entire cross-section of the regenerator.